1. Field of the Invention
The present invention relates to a terminal repeater inserted in a line between an exchange and each terminal device.
2. Description of the Related Art
In a telephone network, private branch exchanges (PBXs) are contained each of which routes incoming calls to desired terminal devices such as telephones and facsimiles by way of telephone lines of wires. In a local network of this type, some terminal devices remotely located often require extremely long telephone lines to connect them to the PBX. In such a case, the telephone lines have large resistance, and the length greatly attenuates speech signals, and consequently degrades speech quality.
To cope with the line loss, repeaters are inserted in long telephone lines to compensate for the line loss. A conventional terminal repeater is provided with a couple of 2 wire-4 wire converting circuits, one installed at the input and the other at the output. The converting circuit contains two paths, incoming and outgoing paths. Amplifiers with AGC functions for compensating the line loss are inserted in the incoming and outgoing signal paths, respectively. A speech signal, for example enters the converting circuit and enters either of the two incoming and outgoing paths. It is amplified by the amplifier contained in the path and leaves the converting circuit to its destination, the PBX or a terminal device.
Thus, the conventional repeater simply amplifies the incoming signal and outputs it to the exterior intact. In the case of a signal containing noise and distortion, these components, together with the signal component, are also amplified. Accordingly, no improvement of the S/N ratio and line quality can be attained. Particularly, when much distortion is contained in DC loop signals utilizing the on-off of the DC loop formed in the communication line, such as seizure signals, off-hook signals, and dial pulses, the PBX mistakenly discriminates those signals one from the other and makes an improper exchange.